1. Field of the Invention
The present invention relates to the technical field of wireless local area network (WLAN) and, more particularly, to a method and system for power saving in a WLAN.
2. Description of Related Art
In the recent years, the IEEE 802.11 wireless local area network (WLAN) has become attractive and popular in the market, due to the benefits of low cost in the network deployment, high bandwidth, and lower technical complexity. The existing Internet applications and service in the application layer can be directly used since the design of IEEE 802.11 WLAN follows the protocol stack of IEEE project-802 standards. For example, the voice over IP (VoIP) service runs in the application layer of user's device to provide the audio service no matter the medium access control (MAC) of data link layer is 802.3 (Ethernet) MAC or 802.11 MAC. For the device that runs VoIP service in the application layer and uses the IEEE 802.11 WLAN as the radio access technology to provide the inexpensive wireless audio service, the requirement of low power consumption in this device becomes a very important issue. The operating time (battery-life per charge) of the user device can be increased if the mechanism of MAC protocol can efficiently save the power.
The IEEE 802.11 WLAN standards had specified the operation of power management. The power-saving mode in the IEEE 802.11 WLAN uses the mechanism of traffic indication map (TIM) to save the power. However, the mechanism of TIM in the IEEE 802.11 standard cannot effectively save the power during the active period.
There are two kinds of the basic network architecture in the IEEE 802.11 WLAN, namely infrastructure WLAN and ad hoc WLAN. In the infrastructure WLAN, there are some access points (APs) in the network. FIG. 1 shows an infrastructure power management operation without point coordination function (PCF). As shown, the AP periodically broadcasts the beacon frame at the start of beacon interval. The power save stations (PS-STAs) that are in sleeping state may periodically wake up to receive the specific beacon frames. Each beacon frame includes the TIM field to indicate those PS-STAs that some packets for the specific PS-STAs are buffered in the AP. The PS-STAs should keep waking up during the remaining beacon interval period to wait for the AP's polling messages during the contention free period (CFP) when that the received traffic indicator bit in the bitmap control sub-field of TIM field is set to 1. In addition, since the broadcast packets or the multicast packets are indicated through the beacon frame with the delivery TIM (DTIM) message, the PS-STAs may wake up at that time to avoid losing the broadcast or multicast packet. FIG. 1 shows that the STA B lost the broadcast frame.
According to the above descriptions, the PS-STAs should keep awake during the remaining beacon interval period to wait for the AP's polling messages whenever they receive its traffic indicator bit in the bitmap control sub-field of TIM field being set to 1. However, this scheme still wastes the energy on unnecessarily monitoring the radio medium, and thus it is not satisfactory in saving the power.
FIG. 2 illustrated the basic power-saving operation in an independent basic service set (IBBS) of an ad hoc WLAN. In an ad hoc WLAN, the PS-STAs periodically wake up at almost the same time and keep awake for a period, namely ad hoc TIM (ATIM) window. In order to synchronize the timing with each other, every PS-STA contends for sending a beacon frame at the start of each beacon interval and the first successful beacon transmission restrains the other STAs from transmitting their beacon frames. Any STA that wants to transmit the data can deliver an ATIM frame to the destination during the ATIM window after the beacon frame. The destination PS-STA must be awake and reply an ACK frame during the ATIM window. After the two-way handshaking, the source PS-STA and the destination PS-STA must keep awake during the remaining beacon interval. The source PS-STA would transmit the data by using the carrier sense multiple access with collision avoidance (CSMA/CA) protocol after the ATIM window (during the remaining beacon interval).
According to the descriptions mentioned above, the source PS-STA and destination PS-STA should keep awake during the remaining beacon interval period to finish the data transmission. However, this scheme still wastes the energy on unnecessarily contending or monitoring the radio medium, and thus it is not satisfactory in efficiently saving power.
U.S. Pat. No. 5,560,021 discloses a power-saving mechanism that is similar to the traditional IEEE 802.11 WLAN. The network establishes a plurality of recurring time intervals, and each time interval has a first portion during which all user devices are in the active mode of operation and a second portion for communication of packet data. The user device having data to transmit should transmit an indicator signal in the first portion. On the other hand, the user device should remain in an active mode of operation if the received indicator signal is addressed for receiving a fragment of a packet of data. However, in the second portion of time interval, the source and destination device need to be in the active mode, which may consume lots of power.
U.S. Pat. No. 5,625,882 discloses an awake counter mechanism to determine whether a user device enters the sleep mode of operation or not. Before entering the sleep mode, the user device selects a synchronization signal rate of 2x, where x is 0<x<8. The user device in sleep mode transfers from the sleep mode of operation to the active mode of operation in order to monitor the synchronization signal's content. The user device enters the active mode of operation if the synchronization signal indicates the request of packet transmission. In this mechanism, each user device needs to be in the active mode for a period no matter the packet transmission has finished, and thus the power consumption cannot be efficiently reduced. Therefore, there is a need to have a novel design of power-saving in the wireless local area network that can mitigate and/or obviate the aforementioned problems.